Electronic devices used for personal and business purposes have become ubiquitous. For example, many persons use personal computers, calculators, entertainment systems, and telephones in their personal and professional lives everyday. Many such electronic devices are portable, and thus include an independent power supply, such as in the form of a battery or batteries.
Batteries used as the foregoing power supplies provide a direct current (DC) source. Commercially available batteries are generally provided in various configurations having predetermined capacities and output voltages. Often times, a battery must be used which provides a DC output voltage different than that required by one or more circuits of an electronic device the battery is to power. For example, some commercially available batteries may provide an output voltage which is too high whereas some commercially available batteries may provide an output voltage which is too low, while no commercially available battery provides an output voltage which is exactly that required by a circuit of a particular electronic device. Similarly, due to such considerations as size (e.g., for enhanced portability), a small form-factor battery may be utilized with respect to an electronic device, although that particular battery does not provide an output voltage required by circuits of the electronic device. Further aggravating this mismatch of battery output voltage to electronic device circuit voltage requirements is the fact that all such batteries tend to suffer voltage sag (decreased voltage output) with time and/or use. Accordingly, even where the output voltage of a battery initially meets circuit voltage requirements, the output voltage of the battery will not likely continue to meet the circuit voltage as the electronic device sees use.
Accordingly, various circuits have been developed to provide DC-DC voltage conversion, such as to increase the output voltage of a commercially available battery (e.g., 1.5 Volt output of a typical AA dry-cell battery) to a voltage sufficient to reliably operate common transistor logic circuits (e.g., 3.3 Volts). For example, DC-DC converters may use a pair of switches (e.g., transistors) to controllably switch current provided from a DC source for converting the source voltage to a higher voltage. In order to prevent there being a short to ground (i.e., “short circuit”), the trigger signals controlling these switches should be synchronous such that both switches are not on (i.e., conducting current) at the same time. Accordingly, such DC-DC voltage conversion circuits have generally employed an oscillator circuit to provide trigger signals for use in producing the desired voltage conversion. See B. Sahu, Gabriel A. and Rincón-Mora “A Low Voltage, Dynamic, Noninverting, Synchronous Buck-Boost Converter for Portable Applications,” IEEE Transactions On Power Electronics, Vol. 19, No. 2, p. 443, March 2004, C. Y. Leung, P. K. T. Mok and K. N. Leung, “A 1-V Integrated Current-Mode Boost Converter in Standard 3.3/5-V CMOS Technologies,” IEEE Journal of Solid-State Circuits, Vol. 40, No. 11, pp. 2265-2274, November 2005, U.S. Pat. No. 6,603,291 to Wheeler et al., U.S. Pat. No. 6,396,250 to Bridge, and U.S. Pat. No. 7,006,364 to Jin et al., the disclosures of which are incorporated herein by reference, for examples of DC-DC voltage conversion circuits employing oscillator circuits.
The oscillators that have heretofore been available for use in DC-DC converters have not been ideal. For example, some previous oscillator configurations implement a fixed delay to provide non-overlapping trigger signal output. The oscillators shown in the above referenced papers entitled “A Low Voltage, Dynamic, Noninverting, Synchronous Buck-Boost Converter for Portable Applications” and “A 1-V Integrated Current-Mode Boost Converter in Standard 3.3/5-V CMOS Technologies” and above referenced U.S. Pat. No. 6,603,291, implement such fixed delays. However, it has been found that such delays, although fixed in the sense that they are not controllably adjustable, are not in fact fixed in the operating environment. For example, the delays are effected by changes in the operating voltage (e.g., power supply voltage sag), by operating temperatures, etc. For example, a “fixed” delay may be 10 nsec at an expected operating voltage of 3.3 Volts, but may increase to 100 nsec at 1 Volt and 200 nsec at 0.6 Volts, for example, such that the delay becomes dominating and the DC-DC converters cannot be reliably switched any longer. The use of mixed circuitry (e.g., analog and digital circuits) aggravates the foregoing problems. As the delays provided in the oscillator circuit change, the trigger signal output becomes asynchronous, resulting in overlapping trigger signal output and failure of the DC-DC converter.
Oscillators implementing programmable delays have been introduced in an attempt to address the foregoing problems. The oscillators shown in above referenced U.S. Pat. No. 6,396,250 implements such programmable delays. These oscillators are typically very complicated, employing both analog and digital circuits, consume appreciable physical space, and are not easily implemented. Moreover, if the programmable delay is in error (e.g., the models used in predicting the needed delay are inaccurate, there are differences introduced to the circuit through integrated circuit process variations, etc.) the trigger signal output may be asynchronous, resulting in overlapping trigger signal output and failure of the DC-DC converter.
To address the foregoing problems, many previous oscillator configurations implement one or more delay tuning circuits. The oscillators shown in above referenced U.S. Pat. No. 7,006,364 implement such delay tuning circuits. Although such tuning circuits may be somewhat effective in controlling delay in response to variations in operating conditions, such as changes in operating voltage, the use of such circuits is not without disadvantage. For example, the delay tuning circuits are typically complicated and add cost, complexity, and physical size to the DC-DC converters. The complexity of delay tuning circuits can result in integrated circuit process variations producing inoperable or unpredictable circuits. Moreover, delay tuning circuits often implement voltage sensors which are ineffective at low voltages (e.g., 0.8 Volts) and thus cannot be used to provide reliable and accurate voltage conversion through much of many battery's life cycle. Such limits on low voltage operation is particularly problematic in the startup of boost or buck-boost DC-DC converter circuits where higher voltages provided by the converter are not available. Additionally, although operating to control delays to maintain non-overlapping trigger signal output, such delay tuning circuits often do not maintain a constant duty cycle with respect to the trigger signal output, and thus the DC-DC converter voltage output is not constant.